Fault control circuit for switched power supply

ABSTRACT

A voltage source, a transformer and a switching controller are coupled for switched mode generation of a regulated output supply voltage. A switching circuit responsive to an on/off signal turns the power supply on and off by establishing a conductive condition in a conductive path. A fault detector establishes a non-conductive condition in a part of the conduction path responsive to an overload condition. A delay circuit establishes a conductive condition in an auxiliary conduction path for a period of time after the power supply is turned on. The auxiliary conduction path becomes non-conductive when the fault detector establishes a conductive condition in the part of the conduction path. The part of the conduction path remains conductive absent an overload condition. A latch arrangement including the delay circuit maintains the non-conductive condition in the auxiliary conduction path until the power supply is turned off.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the field of switched mode power supplies forapparatus having a run mode and a standby mode of operation, such as atelevision receiver. In particular, the invention relates to the fieldof controlling switched power supplies in such apparatus during currentoverload conditions by utilizing a control circuit otherwise present forswitching the power supply, for example an auxiliary power supply, onand off when changing between the run and standby modes of operation.

2. Description of Related Art

In a typical run/standby power supply, for example as used in televisionreceivers, a bridge rectifier and a filter capacitor provide a raw DCvoltage (called the B+ voltage, or raw B+), whenever the power supply iscoupled to the domestic mains. Standby mode loads can be powereddirectly from the B+ voltage or from another voltage that is alwayspresent. Many run mode loads, however, are powered through a voltageregulating supply such as a switched mode supply, that operates only inthe run mode. The run mode power supply for certain loads, such as thedeflection circuits and high voltage screen loads, typically employ theflyback transformer that powers beam deflection. A separate or auxiliarypower supply also can be operated as a switched mode supply and mayprovide a regulated B+ voltage for the flyback transformer as well asother auxiliary supply voltages.

Projection televisions, for example, have particularly demanding powerneeds because they have three high power cathode ray tubes (CRTs). Anauxiliary power supply is useful to power the convergence amplifiers forthe tubes, two such amplifiers generally being required for each CRT.These amplifiers require positive and negative polarity voltages and candissipate substantial power.

In a switched mode supply, an input DC voltage (such as the B+ voltagein a television) is coupled to one terminal of a primary winding of atransformer and the other terminal of the primary winding is coupled toa switching device, such that current is coupled to the transformer whenthe switching device conducts. The switching device is alternatelyturned on and off during the run mode of operation, providingalternating currents in secondary windings of the transformer, which arerectified and filtered to provide run mode supply voltages.

Regulation of the output voltages is achieved by feedback controlprovided by, for example, a feedback winding of the transformer. Therespective secondary windings are closely coupled, so that loadvariations on any of the secondary windings are reflected on thefeedback winding. The feedback control compares a voltage on thefeedback winding with a standard or threshold voltage level, which maybe provided by the switching device, and modulates the frequency and/orpulse width at which the switching circuit is turned on and off. Theswitching device is compensated to render it insensitive to variation ofthe raw B+ input voltage, while maintaining accurate output voltagelevels as current loading varies over a nominal range of powerconsumption.

The switching device for a power supply as described can be anintegrated circuit (IC) power supply controller from the Sanyo STK730series. This controller includes a FET power switching transistor, anerror amplifier and driver, and an over-current protection circuit in asingle package. When coupled into a switched mode supply and firstturned on, current from the B+ voltage flows to ground through theprimary winding of the transformer, the FET and a current sensingresistor. Current increases until the over-current protection circuit inthe controller IC is triggered, whereupon the IC controller turns offits FET power transistor. Energy is transferred to the secondarywindings of the transformer, where the induced AC current is rectifiedand charges filter capacitors. After a starting interval of severalcycles, the output voltage reaches its regulated level. A thresholdcomparison circuit provided by the IC controller is coupled to afeedback winding of the transformer and controls the timing of switchingby the control IC to maintain the regulated output voltage level.Oscillation stabilizes at a frequency and duty cycle that accommodatethe loads coupled to the secondary windings. Many other power supplycontrollers operate in a similar fashion and may be used instead of theSanyo STK730 series.

Such an IC controller will attempt to start whenever the raw B+ voltageis present. Other switched circuits control switching between thestandby mode and the run mode. If during the run mode of operationloading increases on the power supply outputs, the power supply willattempt to provide more current for keeping the feedback winding voltageequal to the control threshold. If a fault condition occurs, such as acurrent overload, the overcurrent fault protection circuit of the ICcontroller, which normally limits current during start-up, becomesoperative to limit the power coupled through the supply. The currentlimiting circuit shuts off the switching transistor before the feedbackcontrol senses that the feedback winding voltage is at the controlthreshold. As a result, the output voltages fall below nominal, toincreasingly lower levels with increased current loading.

Assuming a complete short circuit fault condition on the output, the ICcontroller overload circuit promptly shuts off conduction and littlepower is actually coupled through the supply. However, if there is acurrent overload but not a complete short circuit, substantial power isstill coupled through the supply even as the output voltages drop. Thisis an undesirable operating condition, even a potentially dangerousoperating condition.

SUMMARY OF THE INVENTION

It would be advantageous to shut the auxiliary power supply downentirely when the output is overloaded, as the auxiliary power supplywould be shut down in the standby mode of operation, for example,instead of allowing the controller IC to operate the auxiliary powersupply in an overloaded and/or other fault condition. However, someprovision must be made to permit operation of the current limitingcircuits in the IC controller to enable the power supply to start-up.Otherwise, the low voltage output condition which occurs during powersupply start-up can be incorrectly identified by a fault detectioncircuit to be a low voltage condition resulting from a current overloadfault condition. The auxiliary power supply would never start.

This problem can be solved in an elegant fashion when an auxiliary powersupply is otherwise provided with a switch control for turning theauxiliary power supply on and off as the apparatus changes between thestandby and run modes of operation.

In accordance with an inventive arrangement, such a switch control,which is advantageously coupled to a feedback control signal path, ismodified so as to also be responsive to a fault condition detector, suchas a low voltage and/or overcurrent detector, each of which conditionscan be indicative of a fault condition, such as a short circuit.

In accordance with a further inventive arrangement, a delay circuit isinterposed between the fault condition detector and the switch control,which becomes effective after the auxiliary power supply has been turnedon. The fault condition detector is thereby prevented from disabling theauxiliary power supply for a period of time after the auxiliary powersupply has been turned on, to provide an opportunity for the auxiliarypower supply to establish an operating output voltage without a falseindication of a fault condition.

In accordance with an inventive embodiment, a switched power supply,comprises: a voltage source, a transformer and a switching controllercoupled for switched mode generation of an output supply voltage; afeedback circuit for regulating the switched mode operation responsiveto loading on the output supply voltage; a switching circuit responsiveto an on/off signal for turning the power supply on and off bycontrolling conduction in a conduction path, the power supply beingturned on by a conductive condition in the conductive path; a faultdetector for controlling conduction in a part of the conduction path andestablishing a non-conductive condition in the part of the conductionpath responsive to an overload condition of the output supply voltage;and, a delay circuit for controlling an auxiliary conduction pathbypassing the part of the conduction path controlled by the faultdetector and establishing a conductive condition in the auxiliaryconduction path for a period of time after the power supply is turnedon.

In accordance with this embodiment, the auxiliary conduction pathbecomes non-conductive when the fault detector establishes a conductivecondition in the part of the conduction path, the part of the conductivepath remaining in the conductive condition until detection of theoverload condition.

Further in accordance with this embodiment, the delay circuit also formspart of a latch arrangement which establishes a non-conductive conditionin the auxiliary conduction path after the period of time. The latcharrangement maintains the non-conductive condition of the auxiliaryconduction path until the power supply is turned off.

In accordance with another inventive embodiment, a switched power supplycomprises: a voltage source, a transformer and a switching controllercoupled for switched mode generation of an output supply voltage; afeedback circuit for regulating the switched mode operation responsiveto loading on the output supply voltage; a switching circuit responsiveto an on/off signal for turning the power supply on and off bycontrolling conduction in a conduction path, the power supply beingturned on by a conductive condition in the conductive path; a faultdetector for controlling conduction in a part of the conduction path andestablishing a non-conductive condition in the part of the conductionpath responsive to an overload condition of the output supply voltage;and, a latch arrangement having an auxiliary conduction path in parallelwith the conduction path controlled by the fault detector andestablishing a conductive condition in the auxiliary conduction path fora period of time after the power supply is turned on, the auxiliaryconduction path becoming non-conductive when a conductive condition isestablished in the part of the conduction path, the latch arrangementmaintaining the non-conductive condition of the auxiliary conductionpath until the power supply is turned off.

In accordance with this embodiment, the part of the conductive pathremains in the conductive condition until detection of the overloadcondition.

In each of the embodiments, the latch arrangement comprises: acapacitor; a semiconductor switch having a first junction forming theauxiliary signal path and a second junction providing a charging pathfor the capacitor; and, a diode providing a discharge path for thecapacitor. The first and second junctions of the semiconductor switchbecome non-conductive when the fault detector establishes a conductivecondition in the part of the conduction path, the part of the conductionpath remaining in the conductive condition until detection of theoverload condition.

The capacitor remains charged while the diode is reverse biased. Thecharged capacitor maintains the reverse bias condition in the secondjunction of the semiconductor. The diode becomes reverse biased afterthe power supply is turned off.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an auxiliary power supply having controlcircuitry in accordance with inventive arrangements.

FIG. 2 is a schematic diagram of the auxiliary power supply havingcontrol circuitry in accordance with inventive arrangements andillustrating on/off control in more detail.

FIG. 3 is a schematic diagram of an auxiliary power supply havingcontrol circuitry in accordance with inventive arrangements andillustrating start up and fault detection circuitry in more detail.

FIG. 4 is a schematic diagram of an auxiliary power supply having acurrent overload detection circuit in accordance with inventivearrangements.

FIG. 5 is a schematic diagram of an auxiliary power supply having aquick-reset circuit in accordance with inventive arrangements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 generally shows an inventive switched mode power supply 10 havinga switching controller U1 operable periodically to apply current from avoltage input, for example a raw B+ voltage, to a primary winding W1 ofa transformer T1 for variably coupling power to one or more secondarywindings W2, W3, W4 and W5 of transformer T1. The switching controllerU1 can comprise, for example, a Sanyo STK730 series controller.Switching controller U1 conducts when a driving voltage, for example theraw B+ voltage, is available on its control input CNTL at pin 4.

The raw B+ input supply voltage is a direct current voltage obtainedfrom the output of a bridge rectifier CR1 filtered by a capacitor C1.The raw B+ voltage is present whenever the power supply 10 is coupled tothe domestic mains 22 (i.e., plugged in). However, the power supply 10only operates in a run mode, and is disabled in a quiescent or standbymode.

When power supply 10 is plugged in and is also in the run mode, the rawB+ voltage is present at the control input CNTL of switching controllerU1, thus enabling switching controller U1 to conduct a current throughthe primary winding W1 of transformer T1. The current flow throughwinding W1 induces a voltage across winding W2 of transformer T1, whichvoltage is applied to the control input CNTL through resistor R13 andcapacitor C5. The polarity of winding W2 is such that the voltageinduced across winding W2 keeps switching controller U1 conducting.

Switching controller U1 ceases conducting current through primarywinding W1, or turns off, when the current conducted by switchingcontroller U1 reaches a current limit threshold that is set by thecombination of resistor R14 and capacitor C6. When switching controllerU1 ceases conducting, the magnetic field of primary winding W1collapses, its polarity reverses and the energy contained in primarywinding W1 is transferred to windings W4 and W5, which supply power tothe +15 V and -15 V, respectively, outputs.

As the energy from windings W4 and W5 becomes exhausted, their magneticfields collapse and their polarities reverse. In accordance with thepolarities of windings W2, W4 and W5, winding W2 provides a positivevoltage to pin 4 of switching controller U1, thereby enabling switchingcontroller U1 to once again conduct current through primary winding W1until the current limit threshold of switching controller U1 has beenreached and switching controller U1 ceases conducting current. Energy isthen again transferred from primary winding W1 to windings W4 and W5.This process repeats for several cycles, until the operation of powersupply 10 has stabilized.

Feedback winding W3 controls the duty cycle of switching controller U1after the operation of power supply 10 has stabilized. The voltagedeveloped across feedback winding W3 is compared with an internalreference, equal to approximately -40.5 V, developed by switchingcontroller U1. The duty cycle of switching controller U1 is modulatedsuch that the voltage developed across feedback winding W3 is maintainedapproximately equal to -40.5 V. Feedback winding W3 is coupled to thesecondary windings W4 and W5 so that load changes are reflected in thevoltage developed across feedback winding W3. Thus, feedback winding W3is also used to regulate the output voltages developed by windings W4and W5.

Normally, switching from the standby mode to the run mode or vice versais accomplished under user control via control inputs (not shown) suchas an infrared receiver, panel switches or the like. According to aninventive aspect, additional run/standby switching circuits 36 areprovided to shift the power supply 10 between the operational run modeand the non-operational standby mode. Switching controller U1 requires alarge start-up current. For dependable starting and assistance indeveloping this drive current, the run/standby switching circuits 36include a first circuit 38 coupled between the raw B+ voltage input andthe control input CNTL, for providing a voltage bias to enableconduction by the switching controller whenever the raw B+ voltage inputis present.

In accordance with an inventive arrangement, the drive current biasprovided from first circuit 38 can be shunted away to reduce theavailable drive current to disable the switching controller U1. Thedrive current can be shunted to a source of reference potential, forexample ground.

The run/standby switching circuits 36 further comprise a fault conditiondetection circuit 42 coupled to at least one of the transformersecondary windings W4 and W5. The circuit 42 senses a fault condition,such as current overloading on the auxiliary power supply, for exampleby sensing a low voltage threshold on the output coupled to the same oranother secondary winding W4 or W5. The circuit 42 generates an output41 indicative of a fault condition to disable conduction of switchingcontroller U1 by pulling control input CNTL of switching controller U1to a ground potential, as a means for switching the auxiliary powersupply off, as though the apparatus had been changed to the standby modeof operation. In order to make certain that the startup phase of theauxiliary power supply is not prevented by a false detection of a faultcondition, due to initial low voltage output levels, a delay circuit 40inhibits the effect of the output of the fault condition detectioncircuit 42 for a sufficient period of time for the nominal outputvoltage levels of the auxiliary power supply to be established.

FIGS. 2-5 illustrate in detail different aspect of the inventivearrangements shown generally in FIG. 1. The same reference numbers areused throughout the drawings to refer to the same or comparableelements. Referring to FIG. 2, switching controller U1 is coupled inseries with primary winding W1 of transformer T1. Switching controllerU1 alternately conducts and turns off, for transferring power to thesecondary windings W4 and W5, where the resulting AC signal is rectifiedby diodes D2 and D3 and filtered by capacitors C2 and C3, respectively.The filtered voltages provided on windings W4 and W5 are furtherfiltered by chokes L2 and L3, respectively, to provide operationalsupply voltages +15 V and -15 V, respectively, for powering loads in therun mode.

The polarities of secondary windings W4 and W5 are opposite that of theprimary winding W1, as shown in FIG. 2, such that capacitors C2 and C3are charged when switching controller U1 turns off and the energy storedin the primary winding W1 of transformer T1 is transferred to windingsW4 and W5.

According to an inventive aspect, the power supply 10 as shown isarranged to further control the voltage at control input CNTL ofswitching controller U1 for controlling shifts between run and standbymodes. When the device is in standby mode and switching controller U1 isnot conducting periodically, the only power going into the power supply10 is the raw B+ voltage, which is present because the device is coupledto domestic mains 22. It would be possible in controlling run/standbyoperation to couple and decouple the raw B+ voltage to the switchingelements of the power supply 10 using a relay or other switching devicepowered from a supplemental low power supply (not shown). However,according to the invention a more cost effective solution is obtained byusing a signal derived in part from the raw B+ voltage and in part fromthe run mode voltages, to reduce the bias on control input CNTL toswitching controller U1, namely to bring the voltage on the controlinput to near ground to hold switching controller U1 off until normalbias is restored.

Thus, a voltage divider comprising resistors R1, R2, R3 and R4 iscoupled between the raw B+ voltage and ground, and the junction J1 ofthe voltage divider is coupled to the base of a switching transistor Q2,having its collector coupled to the control input and its emittergrounded. When the raw B+ voltage is present, control input CNTL ispulled to near ground by conduction of transistor Q2. When the powersupply 10 is first coupled to the mains, it is held in standby mode.

The invention is advantageously applied to an auxiliary power supplysuch as the auxiliary supply of a television for powering run mode loadssuch as convergence amplifiers. For switching into the run mode, theinventive power supply senses the presence of a run mode supply voltagedeveloped from a source other than the secondary windings of transformerT1. This run mode supply voltage is compared to a threshold level, andwhen the threshold level is passed, transistor Q2 is turned off,permitting the bias on control input CNTL of switching controller U1 toreturn to normal and permit operation of the auxiliary power supply inthe run mode, namely under feedback control by feedback winding W3 oftransformer T1. For example, the +23 V supply that is developed by therun mode operation of the deflection and other circuits in a televisioncan be used for this purpose.

Referring to FIG. 2, a differential pair of PNP transistors Q3 and Q4have their emitters coupled to the run mode supply voltage by resistorR5, and differentially compare the level of the run mode supply voltage,via the voltage divider of resistors R6 and R7 on the base of transistorQ3, with a reference voltage of +8.2 V provided by Zener diode Z3 on thebase of transistor Q4. When the run mode supply exceeds a leveldetermined by the ratio of resistances in the voltage divider,transistor Q4 conducts and switches on optocoupler U3. Thephototransistor of optocoupler U3 grounds the base of transistor Q2,which ceases conducting, thereby permitting normal bias on control inputCNTL of switching controller U1. Operation of the power supply 10 thencommences in the run mode responsive to the voltages on the secondarywindings W2 and W3 of transformer T1.

Another inventive embodiment is shown in FIG. 3, and includes a latchingcircuit that has the additional function of detecting current overloadconditions, when in the run mode, for switching the power supply 10 intothe standby mode. Current overloading causes the output voltage level todrop below nominal, because in overcurrent conditions the overcurrentprotection circuits of switching controller U1 turn switching controllerU1 off before sufficient power has been coupled through the power supply10 to maintain the nominal output voltage level. This method of currentlimiting is less than optimal for powering loads such as the digitalconvergence amplifiers of a projection television. For such loads, it isadvantageous if the power supply 10 can be turned off when anovercurrent condition occurs, instead of attempting to supply current tothe loads at reduced voltage. According to the invention, this functionis achieved in a manner that interfaces with the circuits controllingswitching between the run and standby modes as in FIG. 2.

In FIG. 3, control for switching from standby to run mode is provided inpart by the run mode supply voltage, such as the +23 V run supply,passing a predetermined voltage as determined by the differentialtransistor pair Q3 and Q4, which provide current to the LED ofoptocoupler U3. The phototransistor of optocoupler U3 then turns offtransistor Q2 and permits operation of switching controller U1.Resistors R1, R2, R3 and R4 provide bias to transistor Q2 at junction J1from the raw B+ supply voltage. In comparison to the embodiment of FIG.2, in which the cathode of the LED in optocoupler U3 is grounded,according to FIG. 3, the current through the LED charges a capacitor C4,through the base of a PNP transistor Q5.

Capacitor C4 provides for a delay upon first switching from the standbymode into the run mode, in which the power supply 10 can start up. Whenthe supply 10 is running and the regulated voltage, in this casenominally +15 V, exceeds approximately +10 V, Zener diode Z4 conductsthrough resistors R8 and R9, and turns on transistor Q6. The currentfrom optocoupler U3 is then shunted to ground through transistor Q6 andcapacitor C4 stops charging. Transistor Q5 is then off and capacitor C4cannot discharge through either transistor Q5 or through diode D6, whichis coupled to the +23 V run mode supply and is reverse biased.

In the event that the +15 V output voltage falls below the level neededto cause Zener diode Z4 to conduct, especially in the case of a currentoverload on secondary winding W4, transistor Q6 turns off due toinsufficient base drive. With transistor Q6 off, capacitor C4 can chargefrom the current through optocoupler U3. When the charge on capacitor C4reaches approximately +10 V, transistor Q5 turns off, and there is nopath for the current through optocoupler U3. In that case, althoughdifferential transistors Q3 and Q4 still detect the presence of the +23V run supply, no current is conducted by the phototransistor ofoptocoupler U3. The raw B+ supply turns on transistor Q2 due to thevoltage divider formed at junction J1 by resistors R1, R2, R3 and R4.The control input CNTL of switching controller U1 is pulled low. Thepower supply 10 shuts off, protecting the loads coupled to the outputs.Thus, unlike a power limiting solution wherein the current limitingcircuits of the switching controller reduce the output voltage belownominal but continue to supply power, the inventive circuit as describedswitches off the power supply 10 in overcurrent conditions. This isaccomplished using the run/standby circuits driven from the raw B+ powersupply, providing a current overload protective function with a minimumof parts and complexity.

As illustrated in FIGS. 1 and 3, fault condition detection circuit 42 isutilized to detect current overload conditions on the +15 V output ofpower supply 10. Detection of overload conditions on the -15 V output iscomplicated by the fact that exclusively positive-polarity biasingvoltages, for example raw B+, are used in power supply 10.

An additional inventive arrangement, shown in FIG. 4, advantageously andelegantly provides for detection of current overload conditions on the-15 V output in the absence of negative-polarity bias voltages.Detection of a current overload condition on the -15 V output, when inthe run mode, causes the power supply 10 to be switched into the standbymode. In FIG. 4, the negative supply voltage overload detection circuit43 is coupled between the +15 V and -15 V outputs of power supply 10.The Zener diode Z6 is biased between the +15 V and -15 V outputs of thepower supply 10, such that the base of transistor Q8 has a bias voltagethat is equal to approximately -2 V when the -15 V output is nominallyloaded. The Zener diode Z6 thus provides a level-shifting mechanism, ora dc offset, that enables the -15 V output to be compared against apositive reference voltage, which in this embodiment is the turn-onvoltage of the base-emitter junction of transistor Q8, for detecting acurrent overload condition.

If, in response to a current overload condition, the -15 V output beginsto drop toward a ground potential, the voltage at the base of transistorQ8 will also tend to move toward ground. Eventually, if the currentoverload condition persists and the -15 V output consequently reaches apredetermined threshold voltage level, the voltage at the base oftransistor Q8 will become positive and will eventually become highenough, for example approximately 0.7 V, to turn on transistor Q8 tosignal a current overload condition. Unlike fault condition detectioncircuit 42, where a current overload condition is signaled by a changein the conductive state of Zener diode Z4, Zener diode Z6 remains in aconductive state when a current overload condition is signaled bytransistor Q8. The desired threshold level can be selected by anappropriate choice of the breakdown voltage of Zener diode Z6.

When transistor Q8 turns on, current is drawn from the base oftransistor Q6, thereby turning transistor Q6 off. Thus, similarly to thedetection of an overcurrent condition on the +15 V output, withtransistor Q6 off, capacitor C4 can charge from the current throughoptocoupler U3. When the charge on capacitor C4 reaches approximately+10 V, transistor Q5 turns off, and there is no path for the currentthrough optocoupler U3. In that case, although differential transistorsQ3 and Q4 still detect the presence of the +23 V run supply, no currentis conducted by the phototransistor of optocoupler U3. The raw B+ supplyturns on transistor Q2 due to the voltage divider formed at junction J1by resistors R1, R2, R3 and R4. The control input CNTL of switchingcontroller U1 is pulled low. The power supply 10 shuts off, protectingthe loads coupled to the outputs.

When the +23 V run supply voltage drops, capacitor C4 is dischargedthrough diode D6, which otherwise would be reverse biased by thepresence of the +23 V run supply. Once capacitor C4 has discharged, thepower supply 10 can be restarted unless there is still an overloadcondition on the output that prevents development of a sufficient outputvoltage to turn on transistor Q6 during the delay time in which thecharge on capacitor C4 can rise to a sufficient voltage to turn offtransistor Q5.

If capacitor C4 is not allowed enough time to fully discharge, forexample if switched mode power supply 10 is shifted from the run mode tothe standby mode and then back to the run mode in rapid succession,transistor Q5 will remain off. The run mode output voltages will thus beprevented from coming up and attaining their nominal output voltagelevels.

A further inventive embodiment shown in FIG. 5 provides a quick-resetcircuit 50 for rapidly discharging capacitor C4 when the +23 V runsupply voltage drops. According to the invention, this function isachieved in a manner that interfaces with the circuits controllingswitching between the run and standby modes as in FIG. 2.

In FIG. 5, the delay circuit 40 has a Zener diode Z5 in parallel withcapacitor C4. When the +23 V run supply voltage comes up, capacitor C4charges through resistor R10 to provide the delay time for the run modeoutput voltages to stabilize at approximately their nominal outputvoltage levels. Zener diode Z5 clamps the voltage across capacitor C4 toapproximately +10 V to prevent damage to the base-emitter junctions oftransistors Q8 and Q9, which are arranged in a Darlington configuration.

Once power supply 10 is in the run mode, transistor Q4 and the diode ofoptocoupler U3 conduct current, in a manner similar to the embodimentshown in FIG. 3. Unlike the embodiment in FIG. 3, however, this currentis not used to charge capacitor C4. The arrangement of transistors Q8and Q9 in a Darlington configuration results in only a minimal currentflow in the base of transistor Q9. Thus, the charging rate of capacitorC4, and the delay time caused thereby, is determined exclusively by thetime constant formed by resistor R10 and capacitor C4. Thisadvantageously eliminates any variation in the charging rate ofcapacitor C4 due to the current amplification factor, or beta, oftransistor Q5 in FIG. 3 or the Darlington arrangement of transistors Q8and Q9 in FIG. 5.

Referring to FIG. 5, when power supply 10 is shifted into the standbymode, the +23 V run supply voltage starts to drop. As the run supplyvoltage drops below a level determined by the ratio of resistances inthe voltage divider of resistances R6 and R7, current flow is redirectedfrom transistor Q4 to transistor Q3. The current flowing throughtransistor Q3 establishes a voltage across resistor R11, which voltagebiases reset transistor Q7 on. Capacitor C4 is thereby rapidlydischarged to ground through resistor R12 and reset transistor Q7 beforethe +23 V run voltage has completely decayed.

It will be apparent to those skilled in the art that, although theinvention has been described in terms of specific examples,modifications and changes may be made to the disclosed embodimentswithout departing from the essence of the invention. Accordingly,reference should be made to the appended claims, rather than to theforegoing specification, as indicating the true scope of the invention.

What is claimed is:
 1. A switched power supply, comprising:a voltagesource, a transformer and a switching controller coupled for switchedmode generation of an output supply voltage; a feedback circuit forregulating said switched mode operation responsive to loading on saidoutput supply voltage; a switching circuit responsive to an on/offsignal for turning said power supply on and off by controllingconduction in a conduction path, said power supply being turned on by aconductive condition in said conduction path; a fault detector forcontrolling conduction in a part of said conduction path andestablishing a non-conductive condition in said part of said conductionpath responsive to an overload condition of said output supply voltage;and, a delay circuit for controlling an auxiliary conduction pathbypassing said part of said conduction path controlled by said faultdetector and establishing a conductive condition in said auxiliaryconduction path for a period of time after said power supply is turnedon.
 2. The switched power supply of claim 1, wherein said delay circuitalso forms part of a latch arrangement which establishes anon-conductive condition in said auxiliary conduction path after saidperiod of time.
 3. The switched power supply of claim 2, wherein saidlatch arrangement maintains said non-conductive condition of saidauxiliary conduction path until said power supply is turned off.
 4. Theswitched power supply of claim 2, wherein said latch arrangementcomprises:a capacitor; a semiconductor switch having a first junctionforming said auxiliary signal path and a second junction providing acharging path for said capacitor; and, a diode providing a dischargepath for said capacitor.
 5. The switched power supply of claim 4,wherein said first and second junctions of said semiconductor switchbecome non-conductive when said fault detector establishes a conductivecondition in said part of said conduction path, said part of saidconduction path remaining in said conductive condition until detectionof said overload condition.
 6. The switched power supply of claim 5,wherein said capacitor remains charged while said diode is reversebiased.
 7. The switched power supply of claim 6, wherein said chargedcapacitor maintains a reverse bias condition in said second junction ofsaid semiconductor.
 8. The switched power supply of claim 6, whereinsaid diode becomes forward biased after said power supply is turned off.9. The switched power supply of claim 1, wherein said auxiliaryconduction path becomes non-conductive when said fault detectorestablishes a conductive condition in said part of said conduction path,said part of said conductive path remaining in said conductive conditionuntil detection of said overload condition.
 10. A switched power supply,comprising:a voltage source, a transformer and a switching controllercoupled for switched mode generation of an output supply voltage; afeedback circuit for regulating said switched mode operation responsiveto loading on said output supply voltage; a switching circuit responsiveto an on/off signal for turning said power supply on and off bycontrolling conduction in a conduction path, said power supply beingturned on by a conductive condition in said conductive path; a faultdetector for controlling conduction in a part of said conduction pathand establishing a non-conductive condition in said part of saidconduction path responsive to an overload condition of said outputsupply voltage; and, a latch arrangement having an auxiliary conductionpath in parallel with said conduction path controlled by said faultdetector and establishing a conductive condition in said auxiliaryconduction path for a period of time after said power supply is turnedon, said auxiliary conduction path becoming non-conductive when aconductive condition is established in said part of said conductionpath, said latch arrangement maintaining said non-conductive conditionof said auxiliary conduction path until said power supply is turned off.11. The switched power supply of claim 10, wherein said part of saidconductive path remains in said conductive condition until detection ofsaid overload condition.
 12. The switched power supply of claim 11,wherein said latch arrangement comprises:a capacitor; a semiconductorswitch having a first junction forming said auxiliary signal path and asecond junction providing a charging path for said capacitor; and, adiode providing a discharge path for said capacitor.
 13. The switchedpower supply of claim 12, wherein said first and second junctions ofsaid semiconductor switch become non-conductive when said fault detectorestablishes a conductive condition in said part of said conduction path,said part of said conduction path remaining in said conductive conditionuntil detection of said overload condition.
 14. The switched powersupply of claim 13, wherein said charged capacitor maintains a reversebias condition in said second junction of said semiconductor.
 15. Theswitched power supply of claim 14, wherein said capacitor remainscharged while said diode is reverse biased.
 16. The switched powersupply of claim 15, wherein said diode becomes reverse biased after saidpower supply is turned off.